1. Field of the Invention
The present invention relates to an optical disk reading device, and in particular, to a method for detecting an unbalanced disk in an optical disk reading device.
2. Description of the Prior Art
The servo-control system of an optical disk reading device (such as a compact disk player) functions to control the position of the objective lens such that when vertical and horizontal excursions of the disk occur due to the rotation of the disk, it is still possible to maintain the focused laser beam on the data layer of the disk to generate an optical reflection signal. This optical reflection signal is the source of the digital signals as well as the feedback signal of the servo-control system.
Ideally, the physical properties of a perfectly circular disk do not change with a change in the position on the disk. For this reason, in theory, rotation of a disk does not in itself cause an excursion in the movement of the disk. However, due to errors that occur during manufacturing, it is very likely that various spots on the disk will have different densities, sometimes even resulting in unique shapes that lead to a non-circular disk. As a result, the centroid of the disk will not coincide with the center of rotation, thereby causing excursions due to the imbalance. In a normal situation, the disk experiences vertical excursions caused by the rotation of the disk, the extent of the focusing error calculated from the optical reflection signal may be used, along with the servo-control system, to generate a control voltage by using a compensator to process the extent of the focusing error. Then, a power amplifier amplifying the control voltage can be used to drive the objective lens (i.e., move the objective lens up and down) to keep the focal point tightly locked on the data layer of the disk.
Under normal conditions, as the disk becomes more unbalanced, a greater the control voltage is needed so that the power amplifier can generate a greater driving force on the objective lens so that data can be read from the disk. However, when the excursion of the disk reaches a certain level, the excursion may cause skipping of tracks, loss of focus and other problems in a optical disk reading device, resulting in failure to properly read data. In addition, because most of the present optical disk reading drives have a high rotational speed of the spindle motor, the noise generated by the friction between the high rotational speed of the unbalanced disk and air will be unacceptable to both the user and the designer. For this reason, a precise and fast identification of the extent of disk excursion, followed by proper adjustment to the maximum speed of the spindle motor, will help achieve a smoother process for data retrieval and reduce the vibration noise of the optical disk reading drive to a tolerable range.
According to a prior art method for identifying an unbalanced disk, the error signals (such as focus and tracking errors) are generated periodically along with the rotational speed of the spindle motor. If the signal is periodically greater than a preset value, then the disk is determined to be unbalanced. However, this prior art method has a few drawbacks. First, this prior art method requires the continuous detection of the status of the error signal. Second, the reason for a periodic change in the values of the error signals is not limited solely to an unbalanced disk, since scratches on the disk may also cause an erroneous identification, so this prior art method cannot always accurately determine whether a disk is unbalanced.
Thus, there remains a need for a method for detecting an unbalanced disk in an optical disk reading device, which overcomes the drawbacks described above.